The Ultimate Guide to Stainless Steel Surface Finishes

The surface finish of stainless steel affects much more than just its aesthetic &#; performance, reliability, and other factors can be greatly impacted.

Contact us to discuss your requirements of stainless steel sheet design. Our experienced sales team can help you identify the options that best suit your needs.

No matter the application or end-use of the stainless steel, its surface finish should be carefully chosen to best meet the requirements at hand. That's because, in addition to providing the desired visual appeal, the choice of finish affects corrosion resistance, characteristics such as electrical properties, impacts weldability, manufacturing operations, and contributes to various economic considerations.

Some applications, such as superconductor components or flexible metal hoses, demand stainless steel with a specific finish for the end-use product to perform as designed.

It's also important to note that not all suppliers use the same terminology to refer to the same finish. Keep reading to learn the subtle differences between each finish to understand better how suppliers may differ.

What Is a Stainless-Steel Surface Finish?

Surface finish is the visual appearance of the metal and is created through various processing methods, including rolling, polishing, and blasting. These finishes range from dull to bright and include special textures that may be mechanically applied for highly specialized applications.

Several factors can affect surface finish, including the production route, material thickness, the surface finish of the rolls, cleanliness, and mechanical abrading.

Most melt mills can produce all sheet and strip finishes. While some finishes (BA and polished) require specialized lines, most mills have these or a third-party processing partner. However, melt mills won't always be able to meet each customer's unique Ra (surface roughness) requirements, and a precision re-roller may be needed.

What Are the Different Types of Stainless-Steel Surface Finishes?

Surface finishes are commonly identified by standard industry designations based on process routing (not appearance) and are different for sheet (greater than 24&#; wide) and strip (less than 24&#; wide). However, as previously mentioned, some suppliers use proprietary designations or different industry standards, so it&#;s best to double-check that you&#;re on the same page.

Ulbrich's surface finish designations are based on ASTM A480 specifications. Below is an overview of the types of finishes available on our sheet products.

Mill Finishes

Whether hot or cold rolled, these finishes are the basic supply condition for all stainless-steel sheet products. They are universally used for many applications and serve as the starting point for mechanical polishing and other finishing processes. To maximize corrosion resistance, most are cleaned with acid (pickled) to remove scale that builds up during hot rolling and annealing (a bright annealed finish being the exception!)

No. 1 Finish

A No. 1 Finish is produced via hot rolling, then annealing and pickling the material in acid. This results in a dull finish and is typically used when a smooth decorative finish isn&#;t important, for example, in elevated temperature applications.

No. 2D Finish

A No. 2D Finish is produced via cold rolling, then annealing and pickling the material in acid. The finish is smooth, non-reflective, and dull. It is ideal for deep drawing applications and is found in auto exhaust components, hardware, and chemical equipment.

No. 2B Finish

A No. 2B Finish is cold rolled, annealed, and pickled, followed by a light temper pass using polished rolls. The finish is smooth, somewhat reflective, and bright. It&#;s a general-purpose cold-rolled finish used in cookware, small tanks, and pharmaceutical equipment.

Bright Annealed (BA) Finish

A Bright Annealed Finish is cold rolled, then annealed in a controlled, protective atmosphere to prevent scale formation during annealing. This finish is not pickled in acid because a thin oxide film is formed during annealing, eliminating the need for pickling. It is used in trim appliances, surgical instruments, and cookware.

Mechanically Polished Finishes

These finishes involve using abrasive materials that effectively cut the steel's surface to some degree.

No. 3 & No. 4 Finishes

No. 3 and No. 4 finishes are produced by mechanically polishing a No. 2B finish substrate. These finishes have uniform appearances that are produced by an emery cloth belt used for polishing. Each finish has a corresponding grit necessary to create its surface texture.

These finishes are among the most used in stainless steel. Applications featuring No. 3 finishes include architectural and food processing components. Applications featuring No. 4 finishes include architectural wall panels, elevators, sinks, and restaurant equipment.

No. 6 Finish

A No. 6 finish is created by Tampico brushing a No. 4 finish. It&#;s dull, silver-white, and less reflective than a No. 4 finish. This finish was once commonly found on stainless steel architectural components up to the s but is much less used today.

No. 7 Finish

A No. 7 finish is produced by buffing a finely ground surface. The finish is high luster with grit lines still visible on the material. The result is highly reflective, almost mirror-like. Some common applications for No. 7 finishes include column covers, ornament trim, and wall panels.

No. 8 Finish

This is produced by polishing with successively higher grit abrasives and then buffing for a mirror finish. By ASTM standards, they are the most reflective of all finishes.

Because they are so reflective, No. 8 finishes have a lot of notable everyday applications, including press plates, signs, and wall panels. Most notably, Chicago&#;s &#;Bean&#; is stainless steel with a #8 finish.

Other Finishes

Custom Finishes

Custom metal surface finishes allow for specific characteristics to be unlocked that are not available if a standard finish is specified. Engineers for advanced and emerging applications specify the surface roughness in Ra or Rz (see below) that they need to meet their objectives.

For more ss bronze finishinformation, please contact us. We will provide professional answers.

For custom surface finishes beyond those mentioned here, contact us to connect with a metallurgical engineer who can assist.

TR (Temper Rolled) Finish

A TR finish is produced when an annealed surface is cold rolled to obtain mechanical properties. The appearance varies based on the alloy and the amount of cold work. Quarter hard, half hard, and full hard are common examples of temper-rolled finishes. This finish is used all the time and is one of the largest categories of stainless steel that we sell at Ulbrich.

Architectural Finish

An architectural finish can be achieved through special finishes at the temper mill, the final reduction stand, special pickle processes, or a combination. These are typically agreed upon between buyer and producer.

What is the Surface Roughness of Stainless Steel?

Any discussion of surface finish would be incomplete without mentioning surface roughness. Surface roughness is a measure of the texture of a manufactured surface. RA or Average roughness is most commonly used in North America. It is the average of the peaks and valleys of a surface over a set sampling length. Values are typically given in microinches or micrometers.

Rz, or mean roughness depth, is most commonly used in Europe and is the average of the highest peak to the lowest valley over 5 sampling lengths.

Surface roughness can have a crucial impact on the durability and performance of the material and must also be closely considered and controlled for.

Still unsure which finish is right for your application? Contact us and speak with someone from our metallurgy or product development team. We also offer custom finishes beyond those mentioned here to accommodate advanced and emerging applications, such as superconductors.

Stainless steel is a commonly used material in applications ranging from medical instruments or chemical storage to transport or power generation because of its high corrosion resistance, hygiene, and strength. While there are more than 3,500 grades of steel, not all stainless steel grades are created equally.

With so many options, how can you ensure you&#;re selecting the appropriate grade for your specific needs?

1. Choose the Right Metal for Your Operating Environment

To determine which grade of steel will uphold best in a given environment, think about the conditions your final product will face. Extremely low pH, high stresses and high temperatures, and crevice corrosion negatively impact stainless steel performance. Steels in the austenitic T3XX series, like the common types 316 and 304 alloys, retain their strength, toughness, and corrosion-resistant properties over the broadest temperature range.

Corrosion resistance is the main reason for choosing austenitic stainless grades. Type 316, with its molybdenum addition, even resists chloride ions found in marine and chemical processing applications. With any steel grade, high-quality structural design is the best defense against corrosion.

2. Prioritize Strength, Ductility, and Toughness

Next, consider these three top mechanical qualities:

• Strength: The stress a metal can withstand before it fractures or deforms
• Ductility: The ability of a material to have its shape changed, such as being drawn out into a wire or thread, without losing strength or breaking
• Toughness: The metal&#;s ability to deform and absorb energy before fracture

Stainless steel contains 10&#;30% chromium as its alloying element, which is what helps it resist corrosion. The nickel addition in austenitic grades provides the highest toughness and ductility among stainless grades. Grades high in chromium, molybedenum, and nickel are the most resistant to corrosion.

Alloy content is not the only aspect to consider when choosing a grade of stainless steel; the material&#;s processing also affects the mechanical response. The duration of time steel is held at different temperatures as part of its cooling process, as well as the total speed at which it is cooled can affect its overall quality.

While the hardness of carbon steels can be increased by heat treatment, austenitic stainless is hardened by cold working operations like rolling, bending, swaging, or drawing at temperatures below the recrystallization temperature. Be aware that increased hardness by cold working operations decreases other properties like elongation and impact resistance.

3. Factor in Form and Process

Austenitic stainless steel is widely available in bar, wire, tube, pipe, sheet, and plate forms; Most products require additional forming or machining before they can be used for their specific application.

Stainless steel tubing, for instance, may need bending or coiling, re-drawing, machining, welding, or end forming. If your stainless steel will see machining processes like CNC machining, drilling, reaming, bevel cutting, chamfering, knurling, or threading, choose a machining rate that mitigates the risk of work hardening or select a &#;free-machining&#; grade containing sulfur.

When welding any stainless steel parts, embrittlement in the weld area is a top concern. Choose a lower carbon grade like 304, 304L, or 316L to reduce carbide formation.

4. Consider Your Customers&#; Preferences

Many designers choose stainless steel for its aesthetic appearance, whether that appearance is a shiny, electropolished &#;bright&#; finish, a dull &#;pickled&#; finish, a matte surface polished to a specific RMS, or a light-absorbing black oxide coating. Austenitic stainless steel grades can take any of these finishes plus the common addition of passivation.

Customers may also need certification for application-specific specifications. For instance, ASTM A213 and A249 should be used for boiler, superheater, and heat-exchanger tubes, while ASTM A908 should be used for hard-drawn austenitic stainless steel industrial needle tubing. There are more than 12,000 ASTM standards, and each addresses a specification so customers know the technical standards tested for chemical composition, heat treatment or temper, and other physical and mechanical attributes.

5. Manage Material Cost and Availability

Although high-performing austenitic stainless steels are the most expensive stainless steels upfront, they are well worth the investment. Choosing a corrosion-resistant material well-suited to its application reduces maintenance, downtime, and replacement costs. Life-cycle costing methods can quantify current and future costs and create an &#;apples-to-apples&#; comparison of different materials.

Selecting a Stainless Steel Supplier

At Eagle Stainless we can help you prioritize your material requirements and guide you toward choosing the perfect stainless steel grades for your application. With quality management certifications in place since , our commitment to quality is evident at every step of the process and designed to help you make the best steel selection for your industrial application.

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